1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), fiat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device). This pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation sensitive material (resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements which serve to generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In the manufacture of flat panel displays, for example, it is often desirable to be able to expose the substrate such that different regions of substrate receive different overall doses of radiation. By so doing it is possible to completely remove a resist on the substrate in some regions, leave the full thickness of resist in other regions and also provide additional regions in which the resist has been partially removed, for example.
This capability is often referred to as half tone or gray tone exposure. This enables a plurality of processing steps in the subsequent manufacturing process to be performed for a single exposure step. For example, a process may be applied to the parts of the substrate that are fully exposed after the exposure. Next a given thickness of resist is removed from those regions which are still covered by resist. This exposes additional regions of the substrate but does not expose those regions where the resist was thickest; subsequently an additional processing step can be performed on only those regions that are now exposed. Finally, all of the remaining resist may be removed before a processing step is performed on the entire substrate.
Gray tone exposure may be produced by individual portions of the substrate being exposed at a given intensity for different amounts of time, by exposing individual portions for the same amount of time but at different intensities (the capability for which is commonly referred to as gray scale exposure) or by a combination of the two.
The present invention provides a method and apparatus for applying different radiation doses to different regions of a substrate.